Lead was used in gasoline to lower combustion temperatures. Hardened valve seats were implemented in engines to withstand the higher combustion temps of unleaded gasoline. Using unleaded gas in a "pre-unleaded" engine can cause valve and valve seat damage. Personally, I would pull the heads and have a machine shop install hardened valve seats and new valves. Shouldn't be more than a few hundred bucks. Until then I would definitely use a lead substitute/additive in the gas.
If you feel this damage has already occurred in your engine, an easy test is to remove the valve covers and rocker arms and lay a straight edge across the tops of the valve stems. Any deviation in valve stem height is a sign of trouble.